Over the past several years, advances in the field of biotechnology have resulted in significant advances in areas such as pharmaceuticals, agriculture, treatment of toxic waste, bioleaching of precious or toxic metals, and removal of sulfur from coal. Further examples of uses of biotechnology with regard to coal can be found in Couch, "Biotechnology and Coal", IEA Coal, Research (1987).
The inability to recover coal that remains after a mining operation is completed is an issue that has received much attention over the past few years. The major reason for this is that over the past fifty years, it is estimated that in the U.S. alone, over 168 quadrillion (10.sup.15) BTU's of unrecovered coal have remained unexploited after conventional mining, much of this in the form of unrecoverable coal gobs. For each year of mining that passes, roughly an additional three quadrillion BTU's of energy resources remain unrecovered. Most often, chemical and mechanical means have been used in attempts to recover the remaining coal, but these methods suffer from various drawbacks which often make them economically unfeasible.
More recently, some attempts have been made to develop biological methods to convert coal to methane. However, these attempts have not been directed to degrading coal in situ, and thus cannot be applied to recovering the vast amounts of coal that remain following a completed mining operation.
Additionally, most recent attempts in the area of biological conversion of coal have been directed to degrading low-rank softer coals, such as lignite, which are less desirable and have a higher moisture content. Examples of such attempts include work by groups such as Arctech, Inc. (see Barik et al, Biological Conversion of Low-Rank Coals, Arctech, Inc., Alexandria, Va.) and Houston Lighting and Power (see Leushner et al, in 13th Biennial Lignite Symposium on Technology and Use of Low Rank Coals, pp. 216-228), who have both disclosed conversion of low rank lignite coal to methane via single step or two step processes involving anaerobic microbes. However, tests involving these microbes on harder, higher rank coals were unsuccessful. These processes also suffer from the drawbacks that because of the slow growth of the anaerobic microorganisms, extremely large reaction vessels are required to produce economic quantities of the desired product, and careful attention must be paid to the environmental conditions for the survival of these organisms. There thus exists the need to develop biological methods of converting high rank coals to methane which can be used in situ in an economical and effective way.
In the patent arts, there has been disclosed the use of large reaction chambers for the gasification of lignite under a primarily two step process as disclosed in U.S. Pat. Nos. 4,845,034 (Menger et al) and 4,826,769 (Menger). These patents relate to a method for degrading lignite coal placed in a subterranean cavity by inoculating the lignite with a culture of microorganisms and allowing the microorganisms to grow on the substrate placed in the cavity, followed by recovery of the biochemical products formed thereby. As in the above described methods, however, the Menger patents suffer from several drawbacks and additionally cannot be used to recover methane from higher rank coals in situ. Among these drawbacks, the Menger method requires that the coal be presolubilized using very high temperatures and alkali solutions. Further, the use of the underground cavity is strictly as a bioreactor and not as a source of coal feedstock. As a result, the coal acted on must be provided to the underground cavity, and coal is not treated in situ. This method requires that surface processing of the substrate materials must be undergone before the reaction can proceed, and this includes finely grinding lignite coal, and making it into a slurry which is then pumped into the subterranean bioreactor. The method also suffers because of its requirement for fast turnover rates on the order of days to achieve an economic operation, and this is difficult because of the slow growth of the anaerobic microorganisms and the achieving of the bioconversion of the lignite coal which is lengthy as well.
It is thus highly desirable to develop a system by which high rank coals such as bituminous coals can be converted into methane using biological methods. It is also highly desirable that a method be found by which an in situ conversion of coals which would otherwise go unutilized be found. This is extremely important because a great deal of coal still remains from abandoned mines or other unminable coal seams which, if converted to methane, could become an extremely important source of energy.